The invention relates to an apparatus and a method for inspecting a thin sample. In particular the invention relates to an apparatus for inspecting a sample using a plurality of charged particle beams, such as a multi-beam scanning electron microscope. The invention may be applied to charged particles of any type, such as electrons, positrons, ions and others.
Such an apparatus is for example disclosed in A. Mohammadi-Gheidari and P. Kruit “Electron optics of multi-beam scanning electron microscope” Nuclear Instruments and Methods in Physics Research A 645 (2011) 60-67. This publication discloses an electron microscope comprising a electron source for generating an array of primary charged particle beams, in particular an array of primary electron beams. These primary electron beams pass an objective lens which directs the electron beams from a common cross-over towards the sample and focuses the primary electron beams into an array of individual spots on the sample. In order to form an image of the sample, it is necessary to detect a characteristic signal from each beam. In an electron microscope this can be either the secondary electron signal, or the backscatter electron signal, or the transmitted electron signal. Methods to detect the secondary electron signal and the backscatter electron signal have been disclosed in respectively PCT/NL2013/050416 and PCT/NL2013/050746.
A disadvantage of those methods is that either the detector itself, or a beam separation device needs to be placed in the path of the primary electron beams. Another disadvantage of those methods is that the excitation of the lenses for the primary beams needs to be adjusted in order to accommodate the signal electron beams. Finally, some contrast mechanisms in the sample to be inspected are better suited for transmission detection than secondary or backscattered detection.
It is an object of the present invention to provide a multi charged particle beam apparatus for inspecting a sample which provides a new detection arrangement for detecting a signal created by the multiple charged particle beams.